The present invention relates to ultrasonic imaging systems for imaging an object for the detection of flaws, defects, internal inhomogeneities and the like.
Ultrasonic imaging is used widely for the detection of flaws, defects, internal inhomogeneities and the like in an object, for example, a welded joint. In principle, ultrasonic imaging involves using an ultrasonic probe to transmit a train of ultrasound pulses towards an object, and to receive the echo pulses reflected therefrom. Changes in the amplitude and/or the travel time of the reflected echo pulses are used to image the flaws, defects, internal inhomogeneities and the like in the object. Several techniques of ultrasonic imaging are known in the art, including A-scan imaging on an oscilloscope, B-scan imaging, C-scan imaging and P-scan imaging (G. S. Passi, "Objectivization of the results of ultrasonic inspection of welding seams," Soviet Journal of Non-destrucrive testing (English language version). 1987; 23 (6): 372-379; G. S. Passi, "Reducing the influence of human factors on the reliability of manual ultrasonic weld inspection," Journal of the British Institute of Non-Destructive Testing. 1995; 37 (10): 788-791; G. S. Passi. "New defect recording system," Journal of the British Institute of Non-Destructive Testing. 1996; 38 (4): 260; U.S. Pat. No. 5,524,627--"Ultrasonic imaging system" to Garri S. Passi, issued Jun. 11, 1996.)
A conventional ultrasonic imaging system, generally designated 10, will now be described with reference to FIG. 1. Ultrasonic imaging system 10 includes an ultrasonic probe 12 for transmitting pulses of ultrasonic energy toward an object under test, and for receiving echo pulses reflected therefrom. Ultrasonic probe 12 is typically a hand-held implement for manipulation by an operator. When the surface of the object to be investigated is inaccessible or irregular, such as an object located adjacent to and between two materials (as in the case of the top bead of a weld 14 located between two plates of metal 15 and 17), an angle ultrasonic probe is employed. The operator grips ultrasonic probe 12 and applies its head to adjacent material 15 in proximity to object 14. Acoustic coupling between ultrasonic probe 12 and adjacent material 15 is facilitated by the application of acoustic coupling fluid to the head of ultrasonic probe 12. The operator manipulates ultrasonic probe 12 over adjacent material 15 according to a probe trajectory determined by the type, size and other parameters of the object to be investigated, such that the linear beam of ultrasound pulses emitted from ultrasonic probe 12 enters object 14 via adjacent material 15, and is reflected back to ultrasonic probe 12. Hence, in order to comprehensively detect variously oriented flaws, defects, internal inhomogeneities and the like within object 14, the operator is required to maintain an appropriate rotational orientation, hereinafter referred to as the swiveling angle, of ultrasonic probe 12 with respect to object 14, while manipulating ultrasonic probe 12 along the necessary trajectory on adjacent material 15.
The location of ultrasonic probe 12 on adjacent material 15, as well as the probe swiveling angle, is determined by a probe location monitoring apparatus 16 which provides real time feedback about the actual trajectory of ultrasonic probe 12 on adjacent material 15 to the operator. The probe location monitoring apparatus 16 includes air acoustic emitters 20 and 22 for transmitting signals, and an air acoustic receiver 24 for detecting the signals. The air acoustic emitters are typically integrated with ultrasonic probe 12 via a probe holder 18. The air acoustic receiver 24 is typically in the form of two flat microphones 26 and 28 placed at right angles to one another, so as to provide a Cartesian coordinate system.
The degree of acoustic coupling between ultrasonic probe 12 and adjacent material 15 is monitored by an acoustic coupling monitoring apparatus 30. The acoustic coupling monitoring apparatus includes a low frequency noise vibrator 32, which continuously emits a low frequency noise reference signal into adjacent materials 15 and 17 and object 14. Acoustic coupling monitoring apparatus 30 determines the degree of acoustic coupling by monitoring the amplitude of the low frequency noise reference signal detected by ultrasonic probe 12.
System 10 further includes a digital computer apparatus 34 for manipulating ultrasound echo data and ultrasonic probe 12 position data.
Digital computer apparatus 34 includes a defect image memory 36 for storing data describing defects in object 14 determined by correlating among the amplitude and time delays of echoes received by ultrasonic probe 12, the coordinates and swiveling angle of ultrasonic probe 12 (as determined by probe location monitoring apparatus 16), and the current degree of acoustic coupling (as determined by acoustic coupling monitoring apparatus 30). A defect image display 38 displays the ultrasound scan image of object 14 depicting defects 40 within object 14 by color coding echo amplitude data retrieved from defect image memory 36.
Digital computer apparatus 34 also includes a probe trace memory 42 for storing position data describing the actual trajectory of ultrasonic probe 12 on the surface of adjacent material 15. The actual probe trajectory, depicted in FIG. 1 on the surface of adjacent material 15 for purposes of illustration only and generally designated 44, includes zones 46 of sufficient acoustic coupling between ultrasonic probe 12 and adjacent material 15 and zones 48 which suffer from an insufficient degree of acoustic coupling between ultrasonic probe 12 and adjacent material 15. The data describing the actual probe trace and the areas of insufficient acoustic coupling are provided for storing in probe trace memory 42 by probe location monitoring apparatus 16 and acoustic coupling monitoring apparatus 30. A probe trace display 50 receives data from probe trace memory 42 and displays an image 52 of the actual probe trace 44 with breaks 54 in the trace indicating zones of insufficient acoustic coupling. Probe trace display 50 also generates perceptible signals indicating the current degree of the acoustic coupling, for example, a label 56, and the current location of ultrasonic probe 12 with respect to object 14, for example, a blinking cursor 58.
Turning now to FIG. 2, a part of ultrasonic imaging system 10 is depicted, including angle ultrasonic probe 12 for imaging object 14. Ultrasonic probe 12 typically includes a scanning ultrasonic crystal 60 connected to digital computer apparatus 34 and to the acoustic coupling monitoring apparatus 30, and additional electrical circuitry, such as a matching coil 62, connected to scanning ultrasonic crystal 60. Acoustic coupling monitoring apparatus 30 determines the adequacy of acoustic coupling by monitoring the amplitude of the low frequency reference signal (originating from low frequency noise vibrator 32) detected by scanning ultrasonic crystal 60. Matching coil 62 resonates electrically at a frequency determined by the nature of scanning ultrasonic crystal 60, so as to suppress signals detected by scanning ultrasonic crystal 60 which do not originate from object 14 (arid are thus non-relevant for purposes of defect image imaging), and thus enhance the sensitivity of scanning ultrasonic crystal 60 to relevant signals returning from object 14. Matching coil 62 may alternatively be located in digital computer apparatus 30, rather than in ultrasonic probe 12, but is still connected to scanning ultrasonic crystal 60.
It is well known that the quality and reliability of an ultrasound examination of flaws, defects, and internal inhomogeneities of an object (such as a metal weld) can be adversely affected by a number of factors. Firstly, as the quality of an ultrasound examination is operator dependent, the overall reliability of an examination is determined by the proficiency of the operator at manually manipulating the ultrasonic probe along an ideal scanning trajectory, while maintaining both an adequate ultrasonic probe swiveling angle relative to the object, and an adequate degree of acoustic coupling between the ultrasonic probe and the object under test. In addition, the reliability of the examination is dependent on the degree of accuracy of probe location monitoring apparatus 16.
The ability of the operator to optimize his/her scanning technique is hampered by the following deficiencies of current ultrasonic scanning systems:
1) Although probe trace display 50 of ultrasonic imaging system 10 indicates to the operator when data fallout has occurred due to poor acoustic coupling, and what the location of ultrasonic probe 12 was at such time, it does not inform the operator when data fallout has occurred due to an inadequate probe swiveling angle. This deficiency hampers the operators ability to efficiently rectify all episodes of data fallout.
2) For acoustic coupling monitoring apparatus 30 to be able to reliably assess the adequacy of acoustic coupling at all locations of ultrasonic probe 12 on adjacent materials 15 and 17, it is necessary that adjacent materials 15 and 17 be fully saturated by the low frequency reference noise emitted by low frequency noise vibrator 32. So as to achieve full acoustic saturation of object 14 and adjacent materials 15 and 17, low frequency noise vibrator 32 is required to emit the reference noise at an appropriate minimum power level, this power level being dependent on the size and nature of the materials being scanned, as well as the type of acoustic coupling fluid being used. Acoustic coupling monitoring apparatus 30 of ultrasonic imaging system 10 typically suffers from poor sensitivity to inadequate acoustic coupling conditions because low frequency noise vibrator 32 emits a low frequency reference noise at an arbitrarily fixed power level, which may often be inappropriate for the scanning conditions. As such, the operator may not be made aware of periods of poor acoustic coupling, or may be erroneously informed that acoustic coupling is inadequate, by acoustic coupling monitoring apparatus 30.
3) The reliability with which acoustic coupling monitoring apparatus 30 of ultrasonic imaging system 10 detects adequate acoustic coupling is typically impaired due to partial suppression, by matching coil 62, of the low frequency reference noise detected by scanning ultrasonic crystal 60. As such, the operator may be erroneously informed, by acoustic coupling monitoring apparatus 30, that acoustic coupling is inadequate.
Probe location monitoring apparatus 16 of current ultrasonic imaging systems suffers from the following sources of inaccuracy:
1) Air acoustic receiver 24 (in the form of two flat microphones 26 and 28 placed at right angles to one another) cannot be adjusted to accommodate objects of different sizes for scanning. Small objects are thus scanned using an unnecessarily large Cartesian coordinate system, which decreases the accuracy of position location.
2) Although airborne ultrasound velocity is influenced by environmental conditions such as air temperature, and thus varies with time and location, air acoustic receiver 24 has a fixed, standardized, calibration for airborne ultrasound velocity. As such, air acoustic receiver 24 cannot be recalibrated to the true local ultrasound airborne velocity at the beginning of each ultrasound scan. This limitation decreases the accuracy of the position location mechanism.
3) Air acoustic receiver 24 (in the form of two flat microphones 26 and 28 placed at right angles to one another) allows for inspection of object 14 from one side only (i.e. adjacent material 15). This is because placing ultrasound probe 12 on the other side of object 14 (i.e. on adjacent material 17) inevitably results in the operators hand being positioned between ultrasound probe 12 and flat microphone 26, thus prohibiting reliable position detection by flat microphone 26. Inspection of object 14 from the opposite side, as is typically required by current inspection standards, therefore entails relocating flat microphone 26 to the opposite side of object 14, performing the second ultrasound scan, and then attempting to correlate the data from the two scans. This process is both time consuming and inaccurate.
4) Because air acoustic receiver 24 is made up of flat microphones 26 and 28, the Cartesian reference system created thereby is accordingly flat as well. As such, when air acoustic receiver 24 is used on curved objects, for example welds in pipes, the position data derived from the flat Cartesian reference system is inaccurate.
An additional deficiency in current ultrasound imaging systems is the fact that defect image memory 36 stores only the most recently acquired echo data for each scanned location on object 14. Consequently, echo data of high amplitude, such as that acquired at an acoustically optimal ultrasonic probe location, will be "overwritten" by echo data from the same location on object 14, but of lower amplitude, such as that acquired when the ultrasonic probe was at an acoustically suboptimal location. Defect image display 38 therefore does not necessarily display the best possible image of the defect.
The above deficiencies reduce the reliability and proficiency of conventional ultrasonic imaging system 10. There is therefore a need for an ultrasonic imaging system for imaging objects for the detection of flaws, defects, internal inhomogeneities and the like, which overcomes the deficiencies of conventional ultrasonic imaging systems